marine drugs Review Collagens of Poriferan Origin Hermann Ehrlich 1,*, Marcin Wysokowski 2, Sonia Z˙ ółtowska-Aksamitowska 2, Iaroslav Petrenko 1 and Teofil Jesionowski 2 1 Institute of Experimental Physics, TU Bergakademie Freiberg, Leipziger str. 23, 09599 Freiberg, Germany; [email protected] 2 Institute of Chemical Technology and Engineering, Faculty of Chemical Technology, Poznan University of Technology, Berdychowo 4, 61131 Poznan, Poland; [email protected] (M.W.); [email protected] (S.Z.-A.);˙ teofi[email protected] (T.J.) * Correspondence: [email protected]; Tel.: +49-3731-39-2867 Received: 30 December 2017; Accepted: 28 February 2018; Published: 3 March 2018 Abstract: The biosynthesis, structural diversity, and functionality of collagens of sponge origin are still paradigms and causes of scientific controversy. This review has the ambitious goal of providing thorough and comprehensive coverage of poriferan collagens as a multifaceted topic with intriguing hypotheses and numerous challenging open questions. The structural diversity, chemistry, and biochemistry of collagens in sponges are analyzed and discussed here. Special attention is paid to spongins, collagen IV-related proteins, fibrillar collagens from demosponges, and collagens from glass sponge skeletal structures. The review also focuses on prospects and trends in applications of sponge collagens for technology, materials science and biomedicine. Keywords: collagen; spongin; collagen-related proteins; sponges; scaffolds; biomaterials 1. Introduction Collagens constitute a superfamily of long-lived extracellular matrix structural proteins of fundamental evolutionary significance, found in both invertebrate and vertebrate taxa. They are among the most studied proteins due to their important functions in mammals, including humans. In addition to their structural function in cartilage and skin formation [1,2], as well as in the biomineralization of hard tissues [3] including bone [4] and dentine [5], collagens are involved in the regulation of diverse cellular functions and processes. During the last 60 years, research into collagens has evolved from the discovery of the structure of collagen [6,7], through studies on its chemistry and biochemistry [8–10], to present-day applications in cell therapy [11], biomedicine [12–14], cosmetics [15], and the food industry [16]. A rod-like triple-helical domain is the typical structural element in all collagens. However, they differ in their size, dislocations of the globular domains and imperfections within the triple helix, self-assembly behavior, and functional roles. The classification of collagens is based on structural and functional features of vertebrate collagens. For example, 28 collagen types have so far been identified and characterized at the molecular level in mammals (see for review [1,17]). Collagens are also divided into subfamilies based on their supramolecular assemblies: fibrils, beaded filaments, anchoring fibrils, and networks [11]. Usually, the amino acid sequences in collagens are responsible for the corresponding functional properties: energy storage capacity, stiffness, or elasticity [18]. Even the type of amino acid motif within the tropocollagen molecule of a collagen can significantly affect its mechanical properties. Consequently, it can be hypothesized that the diversity of collagen polyforms determines their future functions, even within the same organism. Marine vertebrate collagens have attracted scientific attention, mostly as products of fisheries [19]. In particular, fish-sourced collagens from skins and scales [20–22] have been studied and used as alternative collagen sources to avoid the potential risks associated with mammalian collagen due to bovine spongiform encephalopathy and the swine influenza crisis [23]. Mar. Drugs 2018, 16, 79; doi:10.3390/md16030079 www.mdpi.com/journal/marinedrugs Mar. Drugs 2018, 16, 79 2 of 21 In contrast to marine vertebrate collagens, similar structural proteins found in marine invertebrates represent one of the most ancient protein families within Metazoa. Marine invertebrate collagens arose earlier than their vertebrate analogs, and possess diverse unique structural features, including very special structure–function interrelations. Collagens from poriferans, coelenterates, annelids, mollusks, echinoderms, and crustaceans have been discussed in detail in several review papers (e.g., [24–32]) and books (e.g., [33,34]). The limiting factors that have hindered progress in this field of research are the difficulty of purifying marine invertebrate collagens and their relative species-dependent complexity. However, there are more than enough examples in practically every order of marine invertebrates to inspire experts in materials science and biomedicine, especially because the similarities in structure and biosynthesis between vertebrate and invertebrate collagens appear to be more impressive than the differences [24]. Sponges (Porifera) are the most simple and ancient multicellular organisms on our planet, and mostly live attached to a suitable substratum (rock, sandy sediments) on the seabed. Poriferans diverged from other Metazoans earlier in evolutionary history than any other known animal phylum, extant or extinct [35], with the first fossilized sponge remnants found in 1.8 billion-year-old sediments [36–41]. The phylum Porifera is divided into four classes: Hexactinellida, Demospongiae, and Homoscleromorpha, with silica-based skeletons; and Calcarea, with a skeletal network made of calcium carbonates [42]. According to Exposito et al. [27], before the divergence of the sponge and eumetazoan lineages took place, the genes which were responsible for the synthesis of some kind of ancestral fibrillar collagen arose at the dawn of the Metazoa. The duplication events leading to the formation of the A, B, and C clades of the fibrillar collagens occurred before the eumetazoan radiation. Interestingly, the similarity in the modular structure of sponges and humans is preserved only in the B clade of fibrillar collagens. This phenomenon correlates well with the hypothesis of the primordial function of type V/XI fibrillar collagens in initiating the formation of collagen fibrils [27]. Different systems of terminology relating to poriferan collagens are found in the literature, as sponges also display considerable polymorphism with respect to their collagenous structures. The insolubility of most poriferan collagens has been the main obstacle to carrying out any detailed biochemical analysis. Studies on the morphology and nanotopography of the collagenous fibrils have shown that they are dispersed throughout the intracellular matrix within the skeletons of sponges. Cuticular structures have been found in some sponges, but their molecular composition has not been determined [43]. It was accepted very early that collagen fibers in sponges can possess quite different morphological features [44]. Gross et al. isolated two distinct forms of collagen from Spongia graminea, which they called spongin A and spongin B [29]. The first corresponds to fine intercellular collagen fibrils, visible only by electron microscopy. The second, spongin B, forms macroscopically-visible rigid fibers which are characteristic of keratosan demosponges [43]. This was probably the moment when the terminological divergence arose with regard to the term spongin, which was initially proposed by Städeler [45] to denote the skeletal fibrous matter of bath sponges, and was also used for spongins A and B defined by Gross et al. [29]. Up to the present, the authors of numerous publications—especially those on applications of spongin-based scaffolds in tissue engineering [31,46–51]—have used the term collagen for spongin, or even defined spongin as “collagenic skeleton” [52]. Very recently, Tziveleka et al. [53] studied collagen from the marine demosponges Axinella cannabina and Suberites carnosus, and proposed three different terms: insoluble collagen (InSC), intercellular collagen (ICC), and spongin-like collagen (SlC). It is worth noting that the isolation of each form of collagen from demosponges is based on the selectivity of the method used. Data on collagen extraction methods from diverse mineralized sponges (Hexactinellida, Demospongiae) and sponges which lack mineralized skeletons (the subclass Keratosa)—including yields of the extracted collagens—may be found in the relevant papers. In this review, we focus on the structural diversity of collagens and collagen-like proteins in selected sponges, with particular focus on their origin, structural features, and applications in Mar. Drugs 2018, 16, 79 3 of 21 biomedicine and technology, including materials science and biomimetics. The review has the ambitious goal of providing thorough and comprehensive coverage of poriferan collagens (Figure1) as a multifacetedMar. 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